How do you decide what should be patent-able?

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What is the standard for obvious? It’s often been said that all super-villains should have a 5-year-old to point out the obvious flaws in their plans. Child labor laws notwithstanding, how about we hire out some 5-year olds to work for the patent office? Then we have a simple litmus test: hand the patent to the 5-year-olds. If they can understand the concept, then it’s obvious and shouldn’t be a patent.